Conventionally, for example, in welding components such as automobiles, a clamp device is used to clamp the components. In this type of clamp device, the piston of the cylinder portion is displaced in the axial direction under the action of the fluid pressure, so that the clamp arm is rotated through a predetermined angle by a toggle link mechanism connected to the piston rod, for switching between a clamping state in which the workpiece can be clamped and an unclamping state in which the clamping state is released (see, for example, Japanese Laid-Open Patent Publication No. 2001-113468).
Further, in the clamp device disclosed in Japanese Laid-Open Patent Publication No. 2001-113468, the detection portion is connected to the knuckle block connected to the piston rod, and the detection portion is connected to the knuckle block by two proximity sensors provided on the side portion of the clamp body, whereby detecting the clamping state and the unclamping state.